In many power stations or plants a surface condenser passing sea water therethrough has been conventionally utilized for the purpose of cooling the exhaust gas (steam) from a steam turbin for power generating so as to condense it for recycling the same. This type surface condenser has a structure wherein a lots of long condenser tubes, i.e., heat exchanger tubes of small diameter are installed to pass sea water therethrough for cooling the aforementioned steam flowing outside the tubes by the contact heat exchange at the tube walls. Those tubes must be coated on the inner surface thereof with the object of corrosion resistance against sea water. The coating is however strictly conditioned not to deteriorate the heat exchanging performance or function, which is essential to the heat exchanger tubes. It is said that the protective coating of the heat exchanger tubes must be held down in the thickness of the coated film to the order of 10-30.mu. and furthermore uniform so as not to spoil the heat transfer property thereof.
One of the known methods of coating the inner surface of a long tube of small diameter throughout the whole length thereof, flowing of paint into the tube is fairly widely practiced. It is however very inconvenient for a tube already installed in a heat exchanger or the like, because the tube in such a situation can not be inclined in the process of flowing out the remnant paint. Making good use of the air spray coating method in the interior coating of a tube is usually limited to a case wherein the internal diameter of the tube is relatively large, the length of the tube is within 5 meters or so, and the thickness of the coated film is allowed as large as 50-200.mu.. It is therefore employable only for the anti-corrosion coating of a tube used in flowing an ordinary fluid.
In the coating of the heat exchanger tubes (or pipes) for a condenser, so-called condenser tubes, thin and uniform film of coating on the order of 10-30.mu. is required; and it must be executed in tubes of internal diameter as small as 10-40 mm.phi. and of length as large as 5-40 m. Such a situation has conventionally made the thin and uniform interior coating extremely difficult. Besides, the air spray coating method and apparatus was originally developed for the use over a plane place. It is a very excellent method for coating a plane, but applying the same to the interior of a small diametered tube or pipe is very difficult, because it is not suitable for being shifted through the tube inside while uniformly and thinly coating the curved or circular interior surface.
In a spray gun which has been used in the air spray coating, a nozzle portion is said susceptible to sticking of paint and dust at either the paint passage and air passage. Such sticking of paint and dust to the passages is liable to deteriorate the spraying (atomizing) condition due to the clogging thereof. It makes the spraying unstable, which naturally hampers a continuous and uniform coating to be executed smoothly. Besides, the clogging of the essential portions of the nozzle requires a breaking up for cleaning thereof, giving rise to another problem of increasing the man power to be consumed.